Hi Alexander,
Commonly, jBPM 4 used optimistic lock to prevent concurrent thread modify the same record in database. When there were splited execution need to be merged, e.g. in the JoinActivity, jBPM 4 used LockMode.UPGRADE to lock the parent execution. It is means that it will send a SQL likes 'select * from T for update' to lock a record in database and prevent other thread modify this line.
So, if you want to do some integration work, you should consider about using optimistic lock.